I had never heard of “Non-Recognition Aggression Syndrome” in cats until, as the story often goes, one day it happened to me. Or rather, it happened to my two sweet kitties, Trouble and Squirt.

Squirt and Trouble are brothers, littermates who were brought into my life 9 years ago by their feral mama, who somehow decided that my backyard looked like a safe place to set up camp. They were about 10 weeks old when I caught my first glimpse of them.

At first, I saw only their mother, who looked skinny and hungry. I quietly brought out a plate of food for her, which she immediately buried her face into once I had retreated to what she thought was a safe distance. After eating some of the food, I heard her chirp… and out from underneath a hibiscus bush came adorable, hungry little kittens!

Eventually, I adopted and socialized the boys (mama kitty disappeared before I had a chance to catch her). Trouble was a big, confident kitten, with a jet black coat that literally shines blue in the sunlight. Squirt was the runt of the litter, a quiet and gentle gray tabby with beautiful stripes. They were sweet, funny, and inseparable.

That is, until “The Incident”.

My Introduction To Non-Recognition Aggression Syndrome

About 6 months ago, I was walking across the floor in bare feet and happened to step on, of all things, a tooth. As it turned out, it was an intact canine tooth belonging to Trouble that had somehow snapped off at his gumline. As a former Veterinary Technician, my “Oh, $*@&” alarm went off, since a retained tooth root will eventually become infected if it isn’t removed. I immediately made an appointment at the vet for emergency dental surgery.

Squirt and Trouble, before “The Incident”.

After Trouble’s surgery (during which he not only had his tooth root successfully taken out, but also 2 other damaged teeth pulled), I went to pick him up at the vet. Unfortunately, he had been the last surgery on the schedule, so he was still pretty groggy from the anesthetic by the time I got there. Since I had several years of experience recovering veterinary patients who were coming out of anesthetic, rather than leave him there overnight, I was able to take him home to finish waking up.

Here’s where I made my first mistake. After attempting to keep him in a dimly-lit, quiet room for a few hours to finish recovering (during which he freaked out that the door was closed and kept clawing at the door and rubbing his nose under the door crack until the fur literally rubbed off), I decided he was awake enough to go out into the rest of the house with supervision.

Unfortunately, when I opened the door and Trouble bolted out of the room, Squirt was on the other side of the door. I can only imagine what Squirt must have thought when he saw his brother coming straight at him with his fur puffed up, smelling like anesthetic, and looking wild-eyed like Doc Brown from “Back to the Future”. Squirt immediately freaked out, hissed, and ran upstairs to hide.

For the next few days, Squirt avoided Trouble like the plague. The problem was, Trouble wanted his brother for comfort, so he insisted on following Squirt everywhere. This led to a few hissing and growling matches between them, which I thought would eventually subside.

About 2 days later, I was in the kitchen when I heard the most ungodly sound I’ve ever heard coming from the living room. In a matter of seconds, both cats had run upstairs, screaming and slamming their bodies into the wall all the way up. I ran after them for what seemed like an eternity, and by the time I got upstairs, Trouble had cornered Squirt in the bathroom.

There was urine all over the tile floor, with clumps of black and gray fur everywhere. But what I’ll never forget was the look on their faces. Squirt was utterly terrified, laying on his back with his feet in the air, ready to defend himself to the death, while Trouble looked heartbreakingly confused, like he didn’t quite understand what had just happened.

What Is Non-Recognition Aggression Syndrome?

Cats pick up all kinds of strange scents at the vet that they can bring home with them on their coats.

Simply put, Non-Recognition Aggression Syndrome in cats occurs when one cat is inexplicably aggressive to another cat in the household after both have been separated – usually after a trip by one cat to the vet, or after time spent in a boarding kennel. This aggression can lead to vicious attacks against each other, and the aggression can also be redirected to humans in the household (which fortunately did not happen to us).

This behavior, which seems to be unique to cats and does not seem to occur in dogs, is not completely understood, even by veterinary behaviorists. Each case is different, but in most cases, the behavior can escalate to ongoing feuding between the cats, which can lead to the cats’ relationship being permanently broken if something isn’t done to stop it.

Possible Causes of Non-Recognition Aggression Syndrome

There are 2 main theories as to why Non-Recognition Aggression Syndrome occurs, especially in cases when one cat has been to the vet and returns home.

The returning cat looks or acts differently due to anesthesia or sedation. The cat may appear wobbly, or walk or behave differently, which can be alarming to the other cat.

The returning cat, who has been touched by strange humans, or picked up the odors of other animals, disinfectant, iodine, alcohol, or anesthetic gas while at the vet’s, smells differently. Since cats recognize each other by smell first (not by sight), the returning cat can appear to be a total stranger when he comes home. The resident cat then thinks, this may look like my buddy, but they don’t act or smell like my buddy… they must be an intruder!

Sometimes while at the vet, cats can also become so frightened that they involuntarily express their anal glands, which are small sacs located next to the anus. The contents of the anal glands contain strong-smelling pheromones that can linger even after the cat has been cleaned. When the returning cat comes home literally smelling of fear, it can frighten the other cat enough to prompt an attack.

In our case, I believe that Squirt, who is quieter and more shy, became so afraid of Trouble that he began slinking around acting fearful and submissive, which may have somehow actually provoked Trouble to attack him. Cat behavior is complex, and can definitely be difficult to understand!

What To Do If Your Cats Experience Non-Recognition Aggression

First, safely separate the 2 cats immediately. Although you will probably be alarmed, do not yell at the cats or raise your voice. The cat who seems to be the aggressor should be herded into a separate room to give him time to settle down.

How you handle things afterward becomes very important. These strategies can be used to give the cats a chance to recover and, hopefully, return to normal.

Keep the cats separated in different parts of the house, with no visibility to each other, for as long as it takes for them to act calmly again. Be patient – this can take hours, days… or weeks. Each cat should have his own litter box, food and water, and bedding. Never let the cats “work it out” themselves by fighting. This will only lead to more severe, and sometimes permanent, aggression.

During the initial time-out, don’t try to soothe the cats; just leave them alone to give them time to calm down on their own.

Over the next few days, give the cats a chance to reacquaint themselves with each other’s scent without actually seeing each other. You can do this by “scent-swapping”: exchange items that smell like the other cat, such as their beds or toys. You can also take a sock, place it over your hand, and rub one cat for a few minutes, then place the sock on the floor in the other cat’s room. This gives each cat a chance to smell the other without having to actually see them.

During the time-out periods, play with each cat daily and give them both lots of attention. Playing, especially with wand toys, gives them an outlet for their aggression and redirects their focus onto something other than each other.

Gradually reintroduce the cats through a crack in the door, or with the use of a pet or baby gate so they can get a glimpse of each other without being able to have direct physical contact. Whenever the cats see each other without acting aggressively, give them lots of treats and praise so they come to associate good things with being in each other’s presence.

Only when you are certain there will not be any further attacks, allow the cats to be near each other while supervised. If there are any signs of aggression, separate them and try again the next day. BE PATIENT and don’t rush them. Every cat is different.

Daily play with a wand toy can help relax your cat, redirect his energy, and rebuild his confidence after an incidence of aggression.

I’d like to be able to say that this process was quick and easy with Squirt and Trouble. It wasn’t. I had to keep them separated for at least a week, then it took 3 more weeks to gradually re-introduce them. I used a tall pet gate (which was a lifesaver), and used the guest bedroom as a “safe zone”, swapping them in and out of that room until they got used to each other’s smells again. I played with each cat separately for 30 minutes a day while they were apart, and also gave them treats like crazy so they learned that every time they saw each other, they got the best food ever.

Eventually, they were reunited, and thankfully, they are now back to being best friends.

Can Non-Recognition Aggression Be Prevented?

The short answer is, sometimes, but not always. That being said, there are some things you can do if you know you will be taking one cat to the vet.

Make sure that the cat returning from the vet’s office is fully recovered from sedation or anesthesia before coming home.

Keep other cats separated from the returning cat for at least 2 days. This may seem like overkill, but trust me, it works. This gives the returning cat time to lose any strange smells he may have picked up at the vet (especially if he had gas anesthesia, which cats can continue to exhale from their lungs for several hours after surgery), and give him time to settle back into a routine.

If the returning cat will tolerate it, you can try bathing him before reintroducing him to the other cat(s). However, most cats don’t like to be bathed, so it may be more practical to just let the scents wear off on their own. Or you can try using unscented baby wipes (be sure they have zero fragrance added) to wipe him down.

After bathing, rub something from the cat’s regular scent back on him, like a toy or blanket.

Wiser And More Prepared For Next Time

As luck would have it, about 4 months after our episode of Non-Recognition Aggression Syndrome, Squirt also needed to have a tooth pulled. At first, I’ll admit, I freaked out a bit, especially since we had just gotten the household back to normal. But this time, I was ready. When Squirt came home, I kept the boys separated for 3 days, gradually reintroduced them on Day 4, and, thank goodness, everything was fine.

Cats are complex and territorial, and sometimes their behavior can be a bit perplexing. But when we make an effort to see the world through their eyes, it can help us understand what drives them, what they fear, and what we can do to help them overcome some of the challenges of living in our world.

For more valuable tips on how to reintroduce cats, check out this article from cat behavior expert Pam Johnson-Bennett.

Have your cats ever experienced Non-Recognition Aggression Syndrome? If so, how did you handle it? Please share your story with us in the comments below!

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Comments

Wow. I have never heard of this and I’m so sorry you had to experience it! I’ve had cats my whole life so I’m bound to experience this at some point, especially now that I have 3 cats. I’m glad I know what to do now and how to potentially avoid it!

I had never heard of this before either, at least not as an actual syndrome! I had seen the typical suspicious sniffing and avoidance before in my previous cats who had gone to the vet and come home, but never anything like this. It was crazy-scary, and hopefully something I hope to never have to witness again!

This happened to me. I have four cats, one being a maine coon. I took the maine coon to be groomed. Can you believe the SHAVED him??!!! Aside from my be furious, I took him home and one of my other cats became furious too!! I was beside myself. It broke my heart…and made me mad. For one week my maine coon was hissed at (by all the other cats, but worse by just the one) I knew it was going to change the relationships between them. It was a terrible experience. My vet explained just what this writer said. In the future, if one cat needs to go to the vet for something, I will take ALL of them with me (not to be seen by the vet but to just go along for the ride) that way, they all come back smelling like the vets office.

Oh boy… in the case of your Maine Coon, not only did he come back smelling differently, but he looked different too! Hopefully things will calm down after a bit and your cats’ relationships won’t be permanently affected. It can take up to several weeks for things to settle down, so hopefully they will be back to normal soon!

This happened to me this morning, and not after a major “outside” event like a vet visit. I have a shelter-adopted male I got one year before a shelter-adopted female. They were good pals from start. The female never went out, but in his first year, I let the neutered male out until he got sick, and I realized he needed to be indoor only. Well long story short as possible, this morning I gave in to some very insistent meowing and let the male outside again, with a collar. The female went to the door after him, as if curious where the male went. After about a half hour I went out for newspaper and the male came up to me, so I brought him back in. He ate breakfast while the female sniffed him all over. And then she attacked! He ran away and/or cowered. I now have them separated, and I’m hoping things get back to normal soon. But this just shows, non-recognition aggression can happen in a very short time frame. And no, no matter how he meows, the male is never going out again!

Yes, I’ve heard of this happening also! When the returning cat comes home, they also bring with them new smells that can set off other cats in the household. Cat behavior can be so tricky sometimes. :-/

Hopefully after the strange scents wear off of your male kitty, your female kitty will be back to being best buddies with him in no time. And I’m with you, I’d keep him in too! 🙂

I’m having this problem right now. But, neither of mine went to the vet. My cats are basically indoor cats, with an occasional venture out on our deck. It started when they were both startled outside and came running through the door at the same time bumping into each. The sister (Coco) became uncontrollable, hissing, growling, arched back, hair on end and started chasing her brother (Mocha) all over the house, ending up behind my bed screaming and carrying on. I thought it was because she was startled and that she would get over it. That was 3 days ago. I’ve tried to keep them separated and each day let them see each other. The same thing happens all over again. Coco will not tolerated Mocha. Last night Coco (the aggressor) almost completely tore up our carpeting in the guest room! I’m at wits end.

Wow, how incredibly upsetting! It never ceases to amaze me how cats who have gotten along forever can have one incident that sets them off and then all hell breaks loose. 🙁 The only advice I can probably give is to prepare yourself for what might be a longer reintroduction than you hoped for. Since Coco is still reacting so strongly, and it’s only been a few days, if it were me I would try going back to square one and keeping them completely separated for 2-3 days with zero contact. Then very slowly let them smell each other under a door, or do some scent-swapping or room-swapping (put Mocha in a room for awhile, then take him out and let Coco go into the room so she can smell him but not see him). The key is to give them time to get over the trauma first, then get them desensitized to each other. If you can have any kind of interaction without Coco getting upset, go slowly to the next level, and if there is any strong negative reaction, go back to the previous step and try again.

This can be a long process that takes a lot of patience… the incident with my cats literally took over a month for them to be able to be together again 100% of the time, and even after that, it took a few more weeks until they stopped being periodically nervous around each other. But thankfully, after that they were completely back to normal! My best advice is to give your kitties time to forget their “incident”, go very slowly, be patient, and give them lots of love and reassurance (since cats can sense when we’re stressed and will mirror that behavior). When it comes to this type of temporary aggression, patience can really pay off in the end.

This is day 5. I’m doing all the things above, and even went to the vet and got some special food with tryptophan in it and a plug in scent that lets off cat pheromones. That was yesterday. We’ll see how things go today. They haven’t seen each other yet! I have rubbed Mocha with a sock and put it in Coco’s crate at night (along with some catnip). I have to crate her at night because she started ripping up the carpet in the spare bedroom. She has the run of the house during the day, while he hides behind my bed. At night, she is crated and he has the run of the house.

It sounds like you’re doing everything possible to set them up to succeed! Hopefully that, combined with time, patience, and letting them set the pace when it comes to reintegrating with each other, will have them back to being buddies again with no issues. I’m rooting for you, Coco and Mocha!! 🙂

To let u know…ours was a severe case and i personally suffered severe anxiety from it and the recovery process was huge. When we moved house our two saw another cat and then split from each other that very night. They were horrific. We couldn’t get them to tolerate each other at all over the course of a few hours….and we had to end up separating for 9 months!! We took off bedroom door and replaced with a screen door then covered it with b cardboard and slowly cut bit by bit the cardboard away until 4 months later all cardhoard was visible. Each day for that time we swapped them 3 x day so that they were in both spaces and could share scents. We then tried to let them play and eat togther but it took a further 3 months for our most anxious of the 2 to handle that (she was by now om prozac 5mg) By month 8 we were able to put them togthwe and played feather games with them for half hour 2 x day. Then by month 9 my husband decided to put them togther and they had several fairly mundane play fights and scuffles over about 2 hours (to gain heirarchy). None of these scuffles involved yowling or hair raised/ears flat. They were just hisses and deep scowls like hisses. Eventually one retreated from the scuffles and that told us that the ‘top dog’ amongst them had been determined. So then for another 2 months the occasional hiss may have occurred but no major fights. Now it is month 12 and they are back to best pals. I still have severe anxiety if i am concerned about taking to vet etc so when the vet visit was due i just decided to pay the extra to get them to come to my home. Anyway so yah…if anyone needs to know worst case scenario mine could possibly be it. By the way we had the feliway multicat and feliway standard plugged in throughout the house. Also we weaned the one on prozac off about 2 months ago and all is back to normal. So yes please feel free to ask me for help… i have literally slept and walked the reintegration journey for a very long period. I would not wish it upon my worst enemy but i am here to help anyone suffering xo my cats are my all. I would never give them up for anything. If i can get mine back togther so can u all.

WOW. What a nightmare, Natalie!! Yours makes the situation with my boys look like a romp in the park. 🙁 All I can say is that I have incredible respect and admiration for you not giving up on your kitties. Sadly, I know that many people would not have had the patience and determination to go through a reintegration process that lasted for 12 months. And using cardboard and gradually removing it bit by bit is a brilliant idea, I hadn’t thought of that one!

Thank you so much for sharing your success story, and for offering to help anyone going through a particularly difficult reintegration process. For people who have never been through this before, it can be extremely overwhelming, but knowing patience and consistency can finally pay off can give people the encouragement they need to keep trying! Thank you!! 🙂

You give me hope, Natalie! Do you think the prozac helped? My cat, the one who went to the vet and came back aggressive–the vet wanted to put her on prozac for her stress-caused UTIs. I am wondering if that would help with the non-recognition aggression.

We had this issue with our cats a few months ago. Luckily all it took was to keep our girl in the bathroom overnight and have our boy sleep with us and they were fine in the morning! Now our boy had to have knee surgery and has to be contained for 8 weeks! We keep him in our spare bedroom (where there’s no furniture to jump on) and we put some of our blankets in here so he can start smelling normally again. However, we can’t bathe him and he has a cone on his head and is limping from the surgery and our girl won’t come within 5 or 6 feet of him without hissing at him! She even hisses at us if we’ve been petting him and she can smell him on our hands. They stay totally separate during the day and in the evening we try to give them an opportunity to get closer (leave the bedroom door open, give treats when our girl gets closer, give her food in close proximity to the spare bedroom, etc.) but it doesn’t seem to be getting any better and our boy is the least aggressive cat in the world. I think he just misses his best friend 🙁 I’m nervous because the cone can’t come off for over a week when the stitches come out and then we have 6 more weeks of having to keep him in that spare bedroom. I’m afraid by then that their relationship will be irreparable. Any advice?

Hi Andie! Yep, this is a tricky situation. Because your little guy is wearing the cone, not walking normally yet, and probably still feeling under the weather from his surgery, he’s not acting like himself – which unfortunately probably makes him pretty scary-looking to your little girl. 🙁

Since I’m not a feline behaviorist, I can only tell you what I would probably do if I were faced with this situation (keeping in mind, of course, that every cat is different, so you may just want to take it under advisement). Since your boy kitty will not look like himself for another few weeks, I would consider keeping them completely separate for awhile. Every time your girl kitty sees him (or is in close proximity), it just reinforces the fact that he is not himself, and that could be what’s upsetting her. Keeping them separated will give them both a chance to reset a bit. If you choose to try this, you can still do site-swapping – keeping them them from seeing each other, but rotating them in different areas of the house. This will give them the opportunity to smell each others’ scent without the added stress of actually seeing each other. Plus, it will get your boy kitty out into the rest of the house so he doesn’t get depressed from being confined to one space.

Once he is healed up and walking more normally, you’ll probably need to do a full reintroduction similar to what was discussed in the article, almost as if they were strangers. It may be better to go that route (taking as much time as they need) than to risk having them be so upset over the next few weeks that it really does permanently change their relationship. 🙁

I’m currently going through exactly this. My little Olaf had to get surgery to have a tumour removed and as soon as she came home, her sister from the same litter (Lulu) instantly started hissing, growling and spitting at her! I was heartbroken! I’m having to keep them apart because every time Lulu manages to slip back in the room (When visitors leave the door open or if she is sitting waiting outside the door to ambush me when I go to the loo) she makes a bee-line for Olaf and attacks her. They’ve always played happy and it’s horrible to see this now! This has really helped me figure out my next steps though so thanks so much!! Unfortunately Olaf is back at the vet next week to have her stitches removed so I imagine the same thing will happen again *sigh* give me strength!!

Hi Becca! I’m so glad some of these suggestions helped. Don’t give up, mine was a long process too, but the boys are back to being the best of friends with no relapses. It just took time and a LOT of patience! I’m hoping for the same with your kitties! 🙂

I am really hoping for some feedback because my cats are breaking my damn heart…
I have Demeter. She is susceptible to UTIs, and this recent one was caused by stress. Nothing in my house has changed, so I guessed that the stress-source was Cosmo, her “little” brother, who sometimes bullies her.
Demeter had to spend the night at the vet. When she came home, contrary to what the article says, she lunges at Cosmo and hauls after him, both of them screaming and slamming into each other. The vet said to separate them, so I kept Demeter in the guest room for a day.
Last night we tried reintroducing them (after scent exchanges during the day). The appeared fine. We let them out of the room and about 15 min. later they were back at it again.
Cosmo is now in the guest bedroom, I’m trying to get him used to her smell. Demeter is still high strung.
What else should I do? How long did this last for you all? I just want my family back together 🙁

How frustrating! Demeter’s reaction sounds like it might possibly be a case of displaced aggression (since she was the one who went to the vet and came back), but regardless of who started it, unfortunately you’re now dealing with the fallout. 🙁 I think one of the most important things to remember in these situations is to give it enough time to resolve. If you put them back together too soon and they’re not ready, it can undo whatever positive progress you’ve made. It’s tempting to want things to be back to normal quickly, but to do it right can sometimes take weeks (or months). If you’re prepared for a lengthy process and follow their cues, it’s more likely that the outcome will be successful. Patience is always a huge factor in these situations… so prepare for months, and hopefully it will only take weeks!!

You’re very welcome, Annie! I know it’s extremely overwhelming to be in the middle of this situation (I remember having numerous freak-outs wondering if life would ever be normal again), but there is light at the end of the tunnel. Some tunnels are just longer or shorter than others! Hang in there!! 🙂

So, a small update. Since my post, I’ve kept my cats separated, they are doing MUCH better, after a lot of cage therapy (keeping one in an animal crate, let the other walk around), treat time next to each other, and patience. I still don’t trust the little troublemakers by themselves, plus we were going on vacation, so I’ve kept them apart. They are still a little aggressive, but nowhere near that screaming and cat-fight they initially displayed.
Today, Demeter got another UTI, probably caused by stress. My vet thinks it is all behavioral. Demeter’s relationship to Cosmo, their separation, their sometimes aggression…So, she enlisted the help of a professional cat behaviorist at OSU vet medicine center, guess it is that bad!
I guess what I learned is that cats are very very picky and often you can’t see what’s going on beneath the surface. I did not know Demeter was so stressed out, I knew Cosmo picked on her but man, it really did affect her. She’s going back on Prozac, so hopefully that will mellow her out! I mean, where ever you have their food, how many litter boxes, location of litter boxes, safe sleeping places, amount of toys, fresh water, all of that matters, it’s like a damn chemical equation. But it does get better, I still have hope.

Hi Anne! How are your kitties doing now? You are so right in that cats can do a great job of masking what’s simmering underneath. They are also ultra-sensitive to the moods of the people and other pets around them. But even with all of that, I would never have it any other way! Such complicated, mysterious, complex creatures… part of why we love them so much!! 🙂

I’m also experiencing this for the first time. Yesterday, I took Quincy to the groomer to have two mats of fur shaved off. We were in the building all of 10 minutes. The vet’s office is also in the building. When I brought him home, our other cat, Magellan, started hissing and growing. It carried on all day yesterday. I let them stay apart yesterday during the day, then separated them overnight. This morning, I tried rubbing Quincy with a towel then took it to Magellan when I let him out of his room. He acted normal so I let him upstairs. They came within about a foot of each other (I was feeling hopeful!) when he hissed and took off for his high perch. Oh well…I guess I’ll swap rooms today and just keep trying. If I had know this was a possibility, I would’ve taken BOTH of them on the ride since it was a super quick appointment. Since I’m certain they didn’t put any chemicals on Quincy, I’m guessing he released his own “scared” scent and that’s what is upsetting his stay-at-home buddy. Just thought I’d share since that 10 minute visit has caused this issue for us!

Hi Lori! I’m very glad you shared your story, since it doesn’t necessarily take a prolonged vet visit or anesthesia to trigger this situation. The fear response in animals absolutely releases its own scent that can be quite strong, and when other animals detect it, it can actually cause them great stress, presumably because they believe there must be something close by that needs to be feared! Then they can redirect their own fear into an attack on other household members (including humans). Excellent points, thank you so much for sharing!

Pete was severely attacked by a fox or coyote. Its a miracle that he is alive. He is recovering, however an inner ear infection ( or possible permanent damage) leaves him a little unsteady. Plus he is shaved and has a huge scab almost ready to fall off. Pete is now in the house until he and Ci Ci are friends again. But, they will “talk” to each other through the screen door, a different meow than I have ever heard….. more like cooing. I let Pete out under close supervision. Ci Ci will allow him to follow, they stay about 2 feet away. They really WANT to be closer, but if they get into sniffing distance, Ci Ci will hiss and swat, and she was more fierceful when Pete went to drink from what was their shared water bowl. Pete may have permanent difficulties. This will be a challenge to get him back to his home in the barn. I do have hope….. yesterday Ci Ci came up to the screen door and dropped a mouse…. she sooooo wants her old Pete back.

Oooohhhh… this tears at my heart. <3 Although Pete definitely has some obstacles to overcome (not just recovering from his immediate injuries, but potentially suffering some permanent challenges going forward), it sounds like CiCi hasn't given up on him! Although his wobbliness may make him appear "different" for awhile, with time and patience hopefully it will just become the new normal for her and they will be back to being buddies soon. Or better yet, he makes a full recovery and can go back to his normal life!

I’m in the midst of it as we speak: Girlcat just came back from day surgery to remove an abscessed canine and her normally doting, wildly affectionate brother is behaving like a feral cobra. I’d never heard of this before and am alone in the house with them, so am going to have to improvise a second litterbox in order to keep them apart. The reasons for it are what is on the list – she’s visibly wobbly and still stoned, and I have no doubt she smells differently. Could this really damage their relationship permanently? That’s the part that’s terrifying me!!!!

Don’t worry! It will get better! Like all the other posters on here, I know how you feel. Keep them separated, there is no concrete timeline, let your guy cat calm down, let your lady cat recover. My vet originally told me to let them smell each other from under the door. A lot of others here advocate for the good old scent exchange technique. Or Feliway. Most importantly is patience, it may not be fixed overnight, and don’t force anything!

I wish so much I new about this or even thought it was something I needed to look up! I’m currently going through this exact issue… Kitten and adult cat 🙁 Kitten went in for his neuter and when he come home we just let him out of his travel cage I honestly didn’t know any better :(, Adult cat started hissing a growling, its been 24 hours and its still going on (hence me looking online and finding this post) Thank you so much for posting about this now time to implement the strat… wish me luck 🙁

Hay small update, they are so much better. Although Echo (kitten) has moments of biting Wookies neck and pulling out her fur. She is part persion and has stupid amounts of fur which needs daily brushing and now has 2 very noticeable bald patches this wasn’t a issue before the neuter and before the nonrec aggression I have no clue if its tied in with it but that’s the only thing holding them back now. Wookie will growl and get upset doesn’t fight or hurt him she end’s up running. And he just jumps on her and hold’s on with his claws and bites the fur around her neck and pulls it out this happens once to twice a day 🙁 He does stop when we stand up and say no. 🙁

We are going through this right now. We just relocated back home from overseas with our two cats. They are desert kitties from the middle East. They took to my parents dog and all has been well till last night when the neighbors cat jumped onto the windows I’ll and scared my boy cat. He in turn went nuts and bit his sister and went to attack the dog. I have kept the separated since then. He is in the basement with his food, water and litter. He has furniture to sleep on and my son for company. His sister though hisses Everytime she goes near the door. She was confused and so upset that he turned on her. They handled the stress if international travel so well that I cannot stand the thought that this incident can ruin everything. Based on what I’ve read from previous posts I will keep them separated but tomorrow I will keep her in a room with her things and let him roam about the house. I will also use the sock trick and get new toys for them. Is there anything else I can do? Thank you for your help! P.s boy kitty won’t stop crying to come out.

Hi Hillary! I’m very sorry for the delayed response to your question, as I had a family emergency and was unable to respond until now. That definitely sounds like a classic case of redirected aggression (when something scares them and unfortunately they end up taking it out on another family member). How are the kitties doing now? It sounds like your plan was solid, hopefully they are getting along much better!

Thank you for answering back! They are still separated. I visited my vet and his suggestion was rehoming or euthenasia. He was concerned because I have an infant and children in the house. For now I will keep doing what I am doing. I really miss having them out together. It’s very stressful listening to the crying but I cannot put one too sleep. I love them both so much.

Oh my goodness yes, please don’t give up on them! I would never recommend euthanasia for something like this, it often takes time (sometimes many months) for cats to settle in from a move (especially an overseas move), and adding a traumatic incident like what happened to your kitties so soon after they got here would only add to the stress they were probably already feeling. They’ve been through some big changes and definitely deserve the chance to adjust and find their footing! Have you been able to continue to do site-swapping with them, where they take turns with who has access to the main part of the house?

I can only imagine how stressful this must be for you too, especially with having children and an infant – I can’t imagine. 🙁 But I hope you will keep trying with your kitties, time will definitely be your best ally in this situation and I’m hoping that things will slowly start improving and they will be back to being friends again very soon. Please keep me posted, and good luck!!

I swap them every 8 hours or so. Pandora, the girl, sleeps w us so she stays in our room from 10pm till around noon. Then panda, the boy, is put onto our room. We have two windows they can lay in plus toys. My two older boys come in and play with them during their time in the bedroom. It is rough but I am willing to put in the work. I love them both so much.

I have two cats, a male and female. Two days ago i had the female spayed and i let her stay overnight at the vet. When i brought her home, she immediatly ran to the male and he was terrified! He puffed up and ran away. Since then he has been staying away from her, whenever she would go near him, he would hiss and growl and she would just stare at him then continue her buisness (thankfully my male cat is the scardiest cat ever and would never attack another cat, he is easily frightened… he is almost 1 year and a half old while she is 4 months old). Today i put the female in a room and closed it so that the male can use the litter and eat (they share the box and bowls) since for the whole day he hasnt been eating but just the sound of her meowing from the other room was enough to scare him off. I have never heard of this syndrome and now understand whats going on.

Hi Amna, I had never heard of it before either, so I definitely know how you feel! Time and consistency will be your best strategy. There will be good days and not-so-good days, but the better days should start outnumbering the more stressful ones as time goes on. Good luck!!

This is happening to me, my older cat is hissing and swatting at both me and the kitten. I had the kitten at the vet to be neutered and when I came back my older cat is howling and hissing. I have had him in the bathroom with a Feliway diffuser and a litter box, food etc. But he won’t let me touch him or even come too close to him. I’m praying that this situation doesn’t last.

I am SO GLAD I found this site. Now I know I am not alone! My cat Frank (9 years old) had to go in for emergency surgery over the Labor Day weekend. He ate a cassette tape. Thank goodness he is okay but he had 14 stitches on his belly so had to be isolated from my other two cats for 15 days (JJ is 10 yrs old and Spencer is our new kitten, 6 mos old.). I let the other cats in the bedroom supervised for short visits while Frank was recovering. In hindsight, this was probably not a good idea, though there wasn’t any fighting – just a little hissing from JJ (Spencer loves them both, so no problems with her at all). Frank got his stitches out last week and I took a shot at reintroduction right away. At first JJ was tolerant, but not for long. Last night was their worst fight yet – JJ charged Frank and they were locked in a death grip with snarling, clawing, etc. I almost couldn’t get them apart, but I finally did and isolated both of them immediately. All three of us were injured – I have a large cut on my foot and both of the kitties have scratch and bite marks. I was in tears after this happened. My heart was literally breaking. JJ was 1 year old when we got Frank. Frank was abandoned and only 5 weeks old when I adopted him. JJ practically raised him and they have been best buds forever. Now, JJ can’t stand the sight of Frank and I know Frank is wondering why his big brother is attacking him all of the sudden!
After reading everyone’s posts, I realize that I did everything wrong. I also should have realized that there had been two major household disruptions in a matter of 3 months (new kitten, Frank’s surgery). JJ was left with Spencer while most of my attention was on Frank for those 15 days so they have bonded pretty closely. JJ is now very, very protective of Spencer and if Frank even gets close, it sets JJ off. I have now isolated Frank in my room with litter/water/food. I am going to start over VERY slowly. I have Feliway plugged in every room and also have a Feliway spray. I am going to follow the advice of total isolation for probably 2 weeks. I really like the idea of having a gate or something in the doorway so the cats can eventually see each other but not get at each other physically. I also have been neglecting JJ and not giving him as much attention as I should. This is changing today. I am going to schedule play time with all three every day. Hopefully we can all work this out eventually. There is no way I am going to give up on my little furballs!! I love them so much!!
One last thing – is there something I need to be doing differently with Spencer? Should all 3 cats be isolated from each other? Spencer gets along with both JJ and Frank. I was thinking about having her in Frank’s room for a supervised visit and then putting her back out with JJ. Perhaps JJ will smell Frank’s scent on Spencer and start getting used to it? Any help is greatly appreciated. You guys have already given me some great advice!!

I tried to post earlier today and it didn’t show up, so I apologize if it shows up later and this is a duplicate…
I have 3 cats: JJ is 10 yrs old, Frank is 9, and Spencer is 6 months. JJ practically raised Frank. He was 1 year old when I adopted Frank who was abandoned at 5 months old. They have been the best of friends. Now, JJ can’t even look at Frank without attacking and it’s breaking my heart.
Some background: I have encountered non-recognition aggression before after vet visits, but it resolved itself in a matter of hours. This recent experience is much more serious. Frank had to have emergency surgery over Labor Day weekend (he ate a cassette tape). Thank goodness he was okay, but he had 14 stitches in his tummy and had to be isolated when he came home. He had the cone and the stitches and a different smell I’m sure, so I isolated him for several days. The other two cats were curious what was in the bedroom so I allowed short supervised visits which resulted in a few hisses, but no fighting. They would all 3 lay on the bed and relax and eat treats together. Frank’s stitches came out 15 days after surgery and I kept him isolated that day and night, but decided to reintroduce the next day (big mistake). JJ acted increasingly hostile toward Frank. He started stalking him from room to room, but kept his distance. They appeared to all be doing okay so I left them alone for 1 hour (about a week after the stitches came out). I came home to pieces of fur everywhere, pee on the floor and coffee table (as well as some poop), and three very unhappy cats. I felt so bad! How could I have been so stupid! I have isolated since then, allowing short supervised visits outside the bedroom for Frank. I can tell he wants to go to his brother, and looks so hurt when JJ hisses. Last night I couldn’t get to them in time and it was the worst fight yet. They were locked in a death grip, screaming, howling, hissing. I finally got them apart, but not before all 3 of us were injured. I was in tears, heart breaking!!! I isolated them both so they could calm down and I could stop crying. I came upon this site last night after the incident and was so glad to know I wasn’t alone. I see now that I did everything wrong – causing my boys to hurt each other.
I have taken the advice from this posting. I am isolating Frank completely for at least a week. Spencer, the kitten, is scared when the fighting happens but when she is one on one with either of the boys she is playful and loving. Right now, I have JJ and Spencer together and Frank isolated. I really liked the idea of a gate or screen later on so they can see each other but not get at each other physically. I have Feliway diffusers in all rooms as well as Feliway spray. Frank has his own litter box, food, water and toys.
Frank looks so depressed being by himself. This is actually my bedroom so he isn’t alone at night, but he looks so depressed when I leave the room! I am supposed to go on vacation in 3 weeks and hate the idea of isolating him for 6 days straight (I have a sitter coming by each day, but it’s not the same as me being there). My family has had this vacation planned for almost a year so I would hate to back out.
Will I do irreparable damage if I leave for vacation in the middle of this crisis? Should I allow the kitten to visit Frank in the room before I start to reintroduce Frank to JJ? Do you have any advice on my situation? I know I will need to be patient and I know this may take months. I also realize that there were actually TWO disruptions in the household in the last 3 months (new kitten, Frank’s surgery). But I am willing to do anything to get our little family back in order. I love all of them so much!!! Thank you for any advice you can give.

A week ago I took Willow to the vet as I had noticed her abdomen was quite distended. Unfortunately, she has been diagnosed with a large tumor on her liver that inoperable. When I brought her home to spoil her for as long as possible, her sister Violette, began hissing. This has been going on pretty much whenever their paths cross…and isn’t improving. No violence, just hissing. Poor Willow is I’m sure feeling ill from the tumor and now this! Do you think Violette has an idea of what is going on with Willow and is distancing herself?

This also happens with my cats when one goes to the vet because you bring your cat back with a different smell on it from other cats and odors that she’s not familiar with this happened to one of my cats and he wouldn’t have anything to do with my kitten from then on.

I’m going through this right now. We’ve had our kittens since they were 2 weeks old (turned into a “foster fail”). They’re about 6 months old now, and always playing and snuggling, best friends. I had Rosie spayed about a month ago and yesterday Marvin was neutered. When I got home with him, Rosie freaked out and started hissing, growling, and lashing out at him. Poor Marvin just wanted to play with her but she was being extremely hostile. I kept Rosie in a separate room overnight and I felt terrible because she was crying and wanted to be out with me. This morning I opened the door and when she saw Marvin, the hostile behavior started again. They’re separated while I’m at work and I’m sure Rosie is crying. It’s heartbreaking because they were inseparable and now they can’t be in the same room, plus it’s stressful for me because whichever cat is alone at any time is very upset. I’m going to try feliway and slowly reintroduce them but I know it won’t be easy. I just hope they can be friends again soon.

This article gives me such hope! This is exactly what’s happening between my litter sisters, right down to the horrible noises, the chase, the fur flying, and the pee everywhere. We’re working on them little by little, right now they can look at each other through a crack in the door without hissing. It’s been almost 2 weeks since this began. The fact that it took Trouble and Squirt 3 weeks is so reassuring – it means my kitties still have a chance to make up. Thank you so so much for writing this article! No other article on cat aggression helped me 100%, but this one sure did!

I’m going through this now. I have 2 cats. They have been together for 3 years. Anna had dental work done 4 weeks ago and Bristol started attacking her as soon as she got home. I tried just about everything to get them back to their old kitties but they are still not back together. They can be in the same room, Anna usually stays in the top of the cat treehouse. When she comes down, Bristol will usually chase her. But very little growling. It’s better than it was, but Anna is still scared to move around too much.

I wish my vet would have prepared me so I could have prepared my home. So you’re aware, I’m experiencing this as I write. I have a 1 bedroom apartment. With much difficulty, I’m alternating Chloe (in healing) in the living & bedrooms so she gets to alternately enjoy the living room where there’s a terrace & brighter light. The bathroom is where I have to put Miso (the aggressor) while Chloe gets to enjoy more freedom other than just the healing (bed) room. I’ve been wiping Chloe down with unscented baby wipes. After doing that several times, I began wiping Miso down with the same wipes. I thought exchanging scents may help. This will probably begin again in 4 days, when her stitches come out, but this time I’ll be more prepared. What a mess!

Hi Leslie…yes, it can be SO frustrating. 🙁 And I was surprised to find out that unfortunately many vets aren’t aware of how serious and common an issue this is, most likely because people tend to just deal with it on their own and don’t really mention it to their vets. I know that’s how I handled it initially, but then told my vet about it afterwards (when things were back to normal), and he was really surprised. You definitely have a bigger challenge being in a smaller space, but just be patient and give your kitties time to become acclimated again. The trick is not to rush it and not to give up! Hopefully they are doing better now!

Hi Leslie, so sorry for the delayed response, your follow-up comment went into my spam filter and I just now saw it! :-/ I know it’s been a few weeks, so this reply may be too late, but I did seem to have pretty good luck with the diffusers with my guys. I used them in common areas (like where I feed them) as well as in the spaces where I was confining them. I got confirmation that the cats definitely noticed the scent when one day I saw Trouble happily rubbing his face all over the curtains that were hanging right next to one of the diffusers. Since their initial fiasco, I’ve moved into a new house, and I used the diffusers there as well to help them adjust to their new home… and they did great. Hope yours are doing better now too!!

I’m going through this now. We have house cats that we let out in the garden on long leads – usually no problem but the smallest picked up a scent and all hell broke looss!! This happened a few years ago when one escaped and it took two weeks before they would tolerate each other but even though i know they sorted it out before i can’t help but worry this is a permanent thing! i washed them (you can imagine how that went), plugged in the feliway and got the spray, we’ve been keeping them seperated so they can’t see each other and room swapping every few hours / overnight and i’m hoping they will get used to their smells again. I’m planning on letting them see each other in a few days time so fingers crossed. I am so sad and stressed out!They’re my babies and it’s horrible to see them like this with each other. Any more tips much appreciated!!!! Should i room swap less often??? Total mare!!!

Hi Caroline, sorry to hear about your kitties! 🙁 Yes, this scenario is soooo stressful. In my opinion and based on my experiences, you can probably get away with room swapping once or twice during the day and again overnight, you probably don’t need to do it every few hours. And whenever you do let them spend time together, make sure you have lots of treats (or food) that you know they love (for cats, the smellier the food, the better). With cats, sometimes good food covers over a multitude of challenges! Good luck, and please let me know how it goes!!

Great article and what I needed to read right now. We have two male Persian cats that were best of friends. Yesterday we had them both groomed and now our older male (Jack) is attacking our younger guy(Greyson). It seems worse when I’m near Greyson, almost like Jackson seems to want to protect me. We also have a 3 month old so we do r want anyone in danger. Right now we have Jack (the aggressor) in the bathroom with a tall baby gate and his own food and litter. They have seen each other thru the gate and have not hissed. But I’m afraid to try to reintroduce anyone right now. It’s so upsetting to see and we didn’t expect this at all. It’s a mobile groomer and it was very quick for them both but obviously it has changed the dynamic. I’m just praying they resolve it.

Wow, it’s so interesting hearing from everyone what triggered their cats’ aggression…things that I would have never expected! Hopefully your kitties are calming down after this experience and are on their way back to being best buddies again.

About 18 months ago this happened to us. When we first brought our cat, Tabitha into our home almost 12 years ago our other cat Misty would attack her as soon as look at her. But after about three weeks they became best friends. But in June or July 2017 something happened that changed Misty. We have no idea what it was, even to this day. One night, around 2 am-ish, I awoke to bloodcurdling howls and hisses. I raced downstairs to find Misty chasing Tabitha and attacking her. She actually drew blood on her. I had to trap her with a laundry basket as she turned it on me when I tried to catch her. I had to separate them and then try and coax a hissing, terrified tabby cat (Tabitha) from under a computer table. We tried keeping them separate for days and then reintroduce them, and it seemed to be working. Then a week or so later misty attacked again. They’ve been separated ever since. I have tried letting them see each other but not get to each other and misty just loses her s**t at Tabitha. Poor Tabitha is now petrified of other cats and will run in terror from a kitten. We love both our kitties dearly and don’t want to lose either one, but I’m at my wits end over it. We can’t even try to reintroduce them now because Tabitha hisses then runs and hides. It is heartbreaking as we simply don’t know what to do about it. We even thought Misty may have feline dementia and have seen a vet, but nobody wants to believe us or even run tests.

Hi Nicole, I’m so sorry to hear about what’s happening with your kitties. My first thought, knowing how old both kitties are, was that there might be something health-wise going on with Misty? I know you said you’ve seen a vet, but did they do a full exam on her to rule out pain being a possible cause? Oftentimes older kitties can experience pain from arthritis, so a quick full-body x-ray could help rule that out (since arthritic changes usually show up on x-rays). And it’s also possible that, as you mentioned, Misty could be experiencing the onset of dementia. If the first vet didn’t believe you, hopefully you can find another vet for a second opinion – perhaps one who has more experience with older cats. Hopefully you have several more years ahead of you with both Misty and Tabitha, so if there’s something medical causing the issue with Misty, maybe it can be corrected! Good luck, please let me know what happens!

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I'm an author, founder of the Good Pet Parent Blog, and former Registered Veterinary Technician. Through GoodPetParent.com, I share information on pet health, veterinary terminology, animal behavior and communication, training, and nutrition to help pet parents better understand and care for their animal companions. [More.]